Surface-wave electro-acoustic filter

ABSTRACT

An electro-acoustic surface-wave (Rayleigh waves) device comprises a piezoelectric plate equipped with two transducers one at least of which is constituted by two comb-shaped electrodes having interleaved teeth. At least one of the electrodes have teeth of a dissimilar length the envelope of which is the curve representing the Fourier transform of the transfer function of the device when operated as a filter. The device finds application in the fields of radar, telecommunications, data processing, and the like.

Dieulesaint et al.-

3,663,899 [4 1 May 16,1972

[54] SURFACE-WAVE ELECTRO-ACOUSTIC FILTER [72] Inventors: Eugene Dieulesalnt; Pierre l-lartemann,

both of Paris, France 73] Assignee: Thomson-CSF, Paris, France [22] Filed: Apr. 2, 1970 211 App]. No.: 25,158

3,310,761 3/1967 Brauer ..333/30 3,446,975 5/ 1969 Adler et al.

3,150,275 9/1964 Lucy 2,922,965 1/1960 Harrison 2,759,044 8/1956 Oliver ..333/7O X Primary ExaminerHerman Karl Saalbach Assistant Examiner-Marvin Nussbaum Attorney-Flynn & Frishauf 1 ABSTRACT [30] Foreign Application Priority Data An electro-acoustic surface-wave (Rayleigh waves) device comprises a piezoelectric plate equipped with two transducers Apt. 16, France one at least f is constituted two cmb shaped elec trodes having interleaved teeth. At least one of the electrodes U.S. T, 3 have teeth of a length the envelope of which is the [51] Int. Cl R0 H0311 curve representing the Fourier transform of the transfer func- [58] Field of Search 333/7 0 T, Q, 28; 310/ tion of the device when operated as a filter. The device finds BIO/8.1, 8.2, 8.3, 9.7, 9.8;29/25. 35 application in the fields of radar, telecommunications, data 1 processing, and the like. 1 Cited 7 v 12 Claims, 1 Drawing Figure UNITED STATES PATENTS I 3,376,572 4/1968 Mayo ..333/30 X 1 SURFACE-WAVE ELECTRO-ACOUSTIC FILTER The present invention relates to improvements in surfacewave electro-acousticdevices, in particular for filter applications. Electro-acoustic devices are used in processing highfrequency signals, particularly by the telecommunications and radar fields. I

One form of electro-acoustical delay lines is constituted by a plateor wafer of piezoelectric material, for example quartz, to which two transducers are applied, at least one of which is formed of two comb-shaped metal electrodes the teeth of the combed structures being interleaved with one another. Upon application of an electrical signal to one of the transducers, acoustic surface waves, or Rayleigh waves are produced; an electrical signal is then picked up by the other transducer after a delay equivalent to the time taken. by the acoustic waves to propagate from one transducer'to the other. For a detailed description of. the structure and operation of such devices, reference is made to US. Pat. No. 3,360,749, and to literature references there cited.

Each pair of teeth in a comb-shaped transducer exhibits resonance for an acoustic surface-wave whose halfwavelength is equal to the distance between two adjacent teeth. The length of the teeth being substantially higher than the half wavelength the vibratory energy is confined within a narrow radiated beam substantially perpendicular to the length of the teeth. The waves emanating from different pairs of teethhave trajectories of different lengths; consequently, these waves interfere with one another. Thus the transducers are the more selective the larger the number of teeth being used, but the output electrical signals which excite a combshaped transducer are much attenuated if their frequencies deviate from the normal design frequency of the transducer. I

The present invention is directed to eIectro-acoustic elements with comb-shaped, interlaced electrodes which process signals withina determinate frequency band so that the device can be used not only as a delay line but also as a filter.

Subject Matter of the Invention:

The signal input transducer, at least, is constituted by electrodes with a large number of teeth; the teeth of at least one of the electrodes have dissimilar lengths, with a tooth tip envelope having the shape of the Fourier transform of the transfer function presented by the device when operating as a filter.

reference to the accompanying drawings, wherein the single FIGURE illustrates, schematically the electrode arrangements.

A plate. 3 of piezoelectric material (such as quartz, cadmium sulphide, lithium methaniobate, piezoelectric ceramic, etc. etc. has two transducers l and 2 applied to one face. The transducers are spaced from one another and respectively made up of two metal electrodes 1A, 1B and 2A, 28, formed of thin layers of metal and comb-shaped in general form, the teeth of the two electrodes of one transducer being interleaved with those of the other. One of the transducers 2 has the conventional form used in surface-wave (acoustic) delay lines; in this transducer 2, the teeth are equidistant from one another and of equal length and can be small in number, there being provided for example. In accordancewith the invention, the other transducer 1 has teeth which, while equidistant are more numerous, numbering for example 100 or more; the teeth of one of the electrodes 1A of this transducer are equal in length but those of the other, 15, have difierent lengths and The invention will described byway of example with as lying between two limits F1 and F2 situated to either side of a center frequency Fo, the latter being the resonance frequency of the transducer 1 having the larger number of teeth and having electrode 18 whose teeth are dissimilar in length.

The filter effect which can be obtained in the structure in accordance with the invention derives chiefly from the design of this electrode 18 with the dissimilar teeth.

The envelope describing the tips of the teeth, having the form corresponding to the function sin x/x, results in a structure havinga pulse response defined by a wave modulated by sin .x/x, which wave corresponds to the transmission characteristic (or transfer function), of a band-pass filter. The filter effect results from the fact that the pulse response is a function of the geometrical dimensions of the comb electrode structures.

It is well known that the relationship which exists between a transfer function and the pulse responses is given by the Fourier transform, thus, the function sin x/x is the Fourier transform of a rectangular function.

The figure simply illustrates one embodiment of the invention. In another embodiment, both electrodes of one transducer have teeth of variable length; or alternatively, the two transducers may have similar structures, with numerous teeth, at least one electrode group of each of them having teeth which vary in length in accordance with the Fourier transform of the transfer function. v

The envelope defining the teeth of dissimilar length, can have a form other than that described by the function sin x/x. In a general manner, the envelope of the teeth of dissimilar length has a form similar to the Fourier transfonn of the transfer function which it is desired to obtain in operation of the filter device. In the case where several electrodes have teeth of dissimilar length, it is the combination of the envelopes which has the fonn of a Fourier transform. For example, in the embodiment in which the input and output transducars are identical and both have one electrode with teeth of dissimilar length (i.e., in the figure, transducer 1 would be similar to transducer Z), the transfer functionwhich results is the square of the transfer function of one transducer. Numerous classic works on thetheory of filters and signals indicate that Fourier transforms of various functions other than in the example illustrated in the figure the envelope describing the tips of the teeth of the electrode 18, are the shape defined by the function y sinx/x, where y and x are coordinates values corresponding to the reference frame xoy.

The device with the described structure constitutes a delay line of the kind which will delay an electrical pulse proportionally to the distance separating the transducers and, additionally will act as a filter. If a continuous signal'is applied to one of the transducers then thisdevice acts as a bandpass filter capable of passing signals whose frequency F is defined the rectangular, for example triangular, trapezoidal, semisinusoidal, Gauss function, and so on are equally feasible. In accordance with the transmission characteristics which it is desired to impart to the device in accordance with the inven' tion, one or the other of these transforms can be adopted to define the envelope of the teeth.

The pass band increases with decrease in the number of I teeth. The number of teeth is substantially proportional to the quotient of the center frequency F0 and the band width. The

number of teeth should, however, be sufficiently great to define the Fourier transform of the transfer function suffi ciently clearly, which transform generally exhibits several lobes or waves.

The spacing betweenthe teeth determines the center frequency P0 of the device. I

If the Fourier transform exhibits changes of sign (as is the case of the function sin x/x, see the FIGURE), phase reversal can be produced by connecting to the same electrode two adjacent teeth separated by a distance equal to the halfwavelength of the acoustic wave as seen at point 4.

The device can operate at frequencies in the order of several'hundreds megahertz. In the case where the plate 3 is made of quartz, for example, the speed of propagation of the acoustic surface-waves is around 3,200 m/sec.; the wavelength is thus 32 microns at I00 mHz. In-the comb-shaped transducers, two adjacent teeth are spaced apart by half a wavelength; generally speaking, the distance is split equally between the width of a tooth, that is to say the metallized part, and the space between two teeth; up to around I50 ml-Iz, these transducers can be produced by conventional photolithography techniques. Beyond mI-Iz, it is advantageous to employ more sophisticated techniques, for example a method in which the metallized quartz plate is covered with photosensitive resin and exposed to a fine electron beam rather than to actinic light through a mask, as in the photolithographic techniques. The construction of a filter having a predetermined transfer function requires the determination of the lengths and spacing of the teeth. The computation of the spacing of the teeth is quite simple, since the center frequency P of the filter is known. By selecting a substrate, the propagation velocity C of the surface waves along said substrate will be a known quantity from which the wavelength A can be easily derived.

As concerns the lengths of the teeth, it is necessary to compute the pulse response of the filter. This response being the Fourier transform of the desired transfer function, it can be fully detennined as a time function. Assuming that the pulse response is plotted on a time scale, it is very simple to associate to this scale a distance scale, taking into account the known value C of the propagation velocity. The distance scale can then be divided at intervals M2 equal to the spacing of the teeth, and each ordinate value of the pulse response curve will then give the length of the teeth. 7

In another embodiment of the device, certain teeth of the transducers can be omitted, either in order to empirically modify the transmission characteristics of the device or to obtain special effects.

If the piezoelectric plate is a crystal, having X and Y crystallographic axes particular cleavage planes are desirable. For example, if the plate is of quartz, the temperature coefficient can be reduced if an X-cut is used with a direction of propagation making an angle of some 30 with the Y-axis.

We claim:

1. A surface-wave electro-acoustic filter comprising a plate (3) of piezoelectric material;

two transducers l, 2) applied to said plate, at least one of which comprises two metal electrodes (1A, 1B) in combshaped form having interleaved teeth, at least one of the comb-shaped electrodes (18) having teeth of dissimilar length, the envelope described by the tips of the teeth of said electrode being essentially the curve defining the Fourier transform of the transfer function of the device when operating as a filter; and the number of teeth in the electrode with dissimilar length teeth being sufficiently large for the envelope or envelopes to define the major undulations and lobes in said transform.

2. A filter as claimed in claim 1, acting as a bandpass filter, wherein the envelope of the teeth of the combination of the envelopes of the teeth, is approximately the curve following the law described by the function y sin .x/x x and y being coordinates respectively associated with the spacing and the lengths of said teeths.

3. A device as claimed in claim 1 wherein the piezoelectric plate (3) is made of quartz.

4. A device as claimed in claim 3, wherein the quartz plate (3) is X-cut and the electrodes are applied to the plate to obtain a direction of propagation of acoustic waves making an angle of about 30 with the Y axis; X and Y being crystallographic axes of said quartz plate.

5. A device as claimed in claim 1 wherein the piezoelectric plate (3) is made of cadmium sulphide.

6. A device as claimed in claim 1, wherein the pitch of said interleaved teeth is smaller than 10 microns.

7. A device as claimed in claim 1 wherein two adjacent teeth are connected to one and the same electrode at points (4) which correspond to a change in sign in the Fourier transform.

8. A device as claimed in claim 1 wherein more than one of the comb-shaped electrodes has teeth of dissimilar lengths, the combination of the envelopes of the tips of the teeth forming to the plate are equidistant from each other. I

11. A device as claimed in claim 1, wherein the piezoelectric plate (3) is made of piezoelectric ceramic.

12. A device as claimed in claim 1, wherein the piezoelectric plate (3) is made of lithium methaniobate. 

1. A surface-wave electro-acoustic filter comprising a plate (3) of piezoelectric material; two transducers (1, 2) applied to said plate, at least one of which comprises two metal electrodes (1A, 1B) in comb-shaped form having interleaved teeth, at least one of the comb-shaped electrodes (1B) having teeth of dissimilar length, the envelope described by the tips of the teeth of said electrode being essentially the curve defining the Fourier transform of the transfer function of the device when operating as a fIlter; and the number of teeth in the electrode with dissimilar length teeth being sufficiently large for the envelope or envelopes to define the major undulations and lobes in said transform.
 2. A filter as claimed in claim 1, acting as a bandpass filter, wherein the envelope of the teeth of the combination of the envelopes of the teeth, is approximately the curve following the law described by the function y sin x/x ; x and y being coordinates respectively associated with the spacing and the lengths of said teeths.
 3. A device as claimed in claim 1 wherein the piezoelectric plate (3) is made of quartz.
 4. A device as claimed in claim 3, wherein the quartz plate (3) is X-cut and the electrodes are applied to the plate to obtain a direction of propagation of acoustic waves making an angle of about 30* with the Y axis; X and Y being crystallographic axes of said quartz plate.
 5. A device as claimed in claim 1 wherein the piezoelectric plate (3) is made of cadmium sulphide.
 6. A device as claimed in claim 1, wherein the pitch of said interleaved teeth is smaller than 10 microns.
 7. A device as claimed in claim 1 wherein two adjacent teeth are connected to one and the same electrode at points (4) which correspond to a change in sign in the Fourier transform.
 8. A device as claimed in claim 1 wherein more than one of the comb-shaped electrodes has teeth of dissimilar lengths, the combination of the envelopes of the tips of the teeth forming said curve defining the Fourier transform.
 9. A device as claimed in claim 1 wherein the teeth of the comb-shaped electrodes are equidistant from each other.
 10. A device as claimed in claim 7 wherein the teeth applied to the plate are equidistant from each other.
 11. A device as claimed in claim 1, wherein the piezoelectric plate (3) is made of piezoelectric ceramic.
 12. A device as claimed in claim 1, wherein the piezoelectric plate (3) is made of lithium methaniobate. 